Lipoxygenases during Brassica napus seed germination

The peroxidation of polyunsaturated fatty acids is mostly catalyzed by members of the lipoxygenase enzyme family. Lipoxygenase products can be metabolized further in the oxylipin pathway and are known as signalling substances that play a role in plant development as well as in plant responses to wounding and pathogen attack. Apart from accumulating data in model plants like Arabidopsis, information on the relevance of lipid peroxide metabolism in the crop plant oilseed rape is scarce. Thus we aimed to analyze lipoxygenases and oxylipin patterns in seedlings of oilseed rape. RNA isolated from 3 day etiolated seedlings contains mRNAs for at least two different lipoxygenases. These have been cloned as cDNAs and named Bn-Lox-lfl and Bn-Lox-2fl. The protein encoded by Bn-Lox-2fl was identified as a 13-lipoxygenase by expression in Escherichia coli. The Bn-Lox-lfl yielded an inactive protein when expressed in E. coli. Based on Bn-Lox-lfl active site determinants and on sequence homology the Bn-Lox-lfl is most likely a 9-lipoxygenase. Both genes are expressed in light-grown and etiolated cotyledons as well as in leaves. Bn-Lox-2fl protein is more abundant in cotyledons of etiolated seedlings than in cotyledons of green seedlings. Both 13- and 9-lipoxygenase-derived hydroperoxides can be detected during germination. Etiolated seedlings contain more lipoxygenase-derived hydroperoxides in non esterified fatty acids than green seedlings. The 13-lipoxygenase derivatives are 6-8-fold more abundant than the 9-derivatives. Lipoxygenase-derived hydroperoxides in esterified lipids are almost not present during germination. These results suggest that 13-lipoxygenases acting on free fatty acids dominate during B. napus seed germination.

Source: Phytochemistry (2006) vol. 67, p. 2030-2040

Over-expression of chloroplastic lipoxygenase RCI1 causes PR1 transcript accumulation in transiently transformed rice

The acquired resistance state of rice plants, rendering them resistant to subsequent infections with virulent pathogens, can be triggered by treatments with non-host pathogens, necrotizing pathogens and certain chemicals. The 13-lipoxygenase RCI1 (LOX2:Os:2) is induced specifically following treatment with chemical inducers of resistance. Here, we report that the over-expression of RCI1 in a transient transformation assay leads to PR1 transcript accumulation in rice leaves. In addition, we show that this property is due to the enzymatic activity of the transgenic protein. Furthermore, exogenous application of (13S,92,11E,15Z)-13-hydroxy-9,11,15-octadecatrienoic acid (13-HOT), an oxylipin deriving from the reductase branch of the lipoxygenase pathway, is capable of inducing PR1. These results support a role for RCI1 in the generation of signalling molecules during the establishment of the acquired resistance response in rice.

Source: Physiological and molecular plant pathology (2004) vol. 64, p. 37-43

The outcomes of concentration-specific interactions between salicylate and jasmonate signaling include synergy, antagonism, and oxidative...

The outcomes of concentration-specific interactions between salicylate and jasmonate signaling include synergy, antagonism, and oxidative stress leading to cell death

Salicylic acid (SA) has been proposed to antagonize jasmonic acid (JA) biosynthesis and signaling. We report, however, that in salicylate hydroxylase-expressing tobacco (Nicotiana tabacum) plants, where SA levels were reduced, JA levels were not elevated during a hypersensitive response elicited by Pseudomonas syringae pv phaseolicola. The effects of cotreatment with various concentrations of SA and JA were assessed in tobacco and Arabidopsis (Arabidopsis thaliana). These suggested that there was a transient synergistic enhancement in the expression of genes associated with either JA (PDF1.2 [defensin] and Thi1.2 [thionin]) or SA (PR1 [PR1a--glucuronidase in tobacco]) signaling when both signals were applied at low (typically 10–100 µM) concentrations. Antagonism was observed at more prolonged treatment times or at higher concentrations. Similar results were also observed when adding the JA precursor, -linolenic acid with SA. Synergic effects on gene expression and plant stress were NPR1- and COI1-dependent, SA- and JA-signaling components, respectively. Electrolyte leakage and Evans blue staining indicated that application of higher concentrations of SA + JA induced plant stress or death and elicited the generation of apoplastic reactive oxygen species. This was indicated by enhancement of hydrogen peroxide-responsive AoPR10--glucuronidase expression, suppression of plant stress/death using catalase, and direct hydrogen peroxide measurements. Our data suggests that the outcomes of JA-SA interactions could be tailored to pathogen/pest attack by the relative concentration of each hormone.

Source: Plant Physiology (2006) vol. 140, p. 249-262

Enhanced drought tolerance of transgenic rice plants expressing a pea manganese superoxide dismutase

We investigated the role that manganese superoxide dismutase (MnSOD), an important antioxidant enzyme, may play in the drought tolerance of rice. MnSOD from pea (Pisum sativum) under the control of an oxidative stress-inducible SWPA2 promoter was introduced into chloroplasts of rice (Oryza sativa) by Agrobacterium-mediated transformation to develop drought-tolerant rice plants. Functional expression of the pea MnSOD in transgenic rice plants (T1) was revealed under drought stress induced by polyethylene glycol (PEG) 6000. After PEG treatment the transgenic leaf slices showed reduced electrolyte leakage compared to wild type (WT) leaf slices, whether they were exposed to methyl viologen (MV) or not, suggesting that transgenic plants were more resistant to MV- or PEG-induced oxidative stress. Transgenic plants also exhibited less injury, measured by net photosynthetic rate, when treated with PEG. Our data suggest that SOD is a critical component of the ROS scavenging system in plant chloroplasts and that the expression of MnSOD can improve drought tolerance in rice.

Source: J Plant Physiol. (2005) vol. 162, p. 465-472

OsCDPK13, a calcium-dependent protein kinase gene from rice, is induced by cold and gibberellin in rice leaf sheath

Calcium-dependent protein kinases (CDPKs) play an important role in rice signal transduction, but the precise role of each individual CDPK is still largely unknown. Recently, a full-length cDNA encoding OsCDPK13 from rice seedling was isolated. To characterize the function of OsCDPK13, its responses to various stresses and hormones were analyzed in this study. OsCDPK13 accumulated in 2-week-old leaf sheath and callus, and became phosphorylated in response to cold and gibberellin (GA). OsCDPK13 gene expression and protein accumulation were up-regulated in response to GA3 treatment, but suppressed in response to abscisic acid and brassinolide. Antisense OsCDPK13 transgenic rice lines were shorter than the vector control lines, and the expression of OsCDPK13 was lower in dwarf mutants of rice than in wild type. Furthermore, OsCDPK13 gene expression and protein accumulation were enhanced in response to cold, but suppressed under salt and drought stresses. Sense OsCDPK13 transgenic rice lines had higher recovery rates after cold stress than vector control rice. The expression of OsCDPK13 was stronger in cold-tolerant rice varieties than in cold-sensitive ones. The results suggest that OsCDPK13 might be an important signaling component in the response of rice to GA and cold stress.

Source: Plant Mol Biol. (2004) vol. 55, p. 541-552